I still don't understand one thing and maybe someone can make this clear:
I understand reasonaly well the purpose of I, B and P frames.

If I use Tmpg Xpress and I select in the tab GOP structure e.g. "standard" I see this GOP structure "display":
IBBPBBPBBPBBPBBP

In my opinion that are 16 pictures 1xI, 10xB and 5xP.
I use (and set) PAL and I read that the "maximum frame number per GOP " should be 15 (for PAL). How can it be that there are 16 pictures?
Yes, I can change the GOP structure and number of pictures by adjusting the nuber of B and/or P pictures. Then the display will show e.g. IPPPP (when I select 1xI and 4xP pictures).

Still I don't understand this "maximum number of frames per GOP".
Is it right that (when scene detection is set) and a scene change is detected on let say picture 4 the stream will be IBBIBBPBBPBBPBBPBBP ?

And what't also confusing: the IBP's are called "pictures" not frames. To which is the "Maximum frame number per GOP" refering to?

I will close with these questions:
What is the purpose of "maximum frame numbers per GOP"?

What will be the effect when you decrease or increase it?

What is the reason that Tmpg doesn't change the GOP structure "display" if you change the Maximum frame number per GOP? (it doesn't matter if you set it to 0 or 15 it still displays IBBPBBPBBPBBPBBP).

Picture and frame refer to the same thing. If you set a limit lower than the number of pictures in your GOP structure, TMPGEnc will use only the first N picture types, and then start a new GOP. TMPGEnc's defaults are for NTSC (used in Japan, where the program is made), so the default structure has more than 15 pictures. As long as you set the limit to 15, the GOPs will only be 15 pictures long.

1. The "maximum frame number per GOP" does hard limit the number of frames per GOP. If you specify more frames (wich can be set by the number of I, B and P frames) then the this limit the extra frames "types" will be ignored. Just as an example: when I set 1xI, 5xP (IPPPPP) and the GOP limit is 4 then the GOP will look like IPPP.

3. The purpose of this maximum GOP size limit is that by using a large value we can gain quality by using more P and/or B frames (who will be small in size and therefore give the I frame more KB left to code).

4. When the limit value is decreased more I frames will be inserted in the stream (every GOP start with a I frame) and therefore the quality will drop (when using the same bitrate).

5. Small GOP sizes will be better when cutting/editing MPG because less frames will be recompiled.

6. To be compatible with old DVD players limit the videobitrate to 8000 kbit/s.

I learned a lot of things the last day's and your postings added a lot to my knowledge about GOP, MPG etc. thanks!

3. The purpose of this maximum GOP size limit is that by using a large value we can gain quality by using more P and/or B frames (who will be small in size and therefore give the I frame more KB left to code).

The puprpose of the GOP limit is to make the video stream compatible with standard players. Technically, a much longer (or virtually infinite) GOP would be ideal, but since the player needs to load the whole GOP to decode each frame, and since the memory on set-top players is pretty small, the GOPs need to be limited to 15 / 18 frames. Many DivX files, for example, have GOPs of more than 100 frames (sometimes as long as 500). This is one of the reasons why DivX achieves much better compression than DVD MPEG-2.

okude wrote:

4. When the limit value is decreased more I frames will be inserted in the stream (every GOP start with a I frame) and therefore the quality will drop (when using the same bitrate).

Assuming you use scene change detection, yes. If you don't, then an I-picture that happens to "land" on a scene change will actually improve your overall quality.

okude wrote:

6. To be compatible with old DVD players limit the videobitrate to 8000 kbit/s.

Yes. This is also the official video bitrate limit for multi-angle DVDs, even on modern players (although some players won't have any problems playing files encoded at 10 Mb/s or even more, but a good authoring program will refuse to accept those files).

Really, it HAS to be limited to 8000? I was under the impression that as long as everything total stayed at 9800 or below it was fine. Since I often use PCM audio, which is 1536, I usually encode the video at 8200 since 8200+1536 is less than 9800.

For multi-angle DVDs, yes, it has to stay under 8000 kb/s, or it won't be "legal". For regular (single angle) DVDs, the total can go up to 9800 but some older players will have problems if the video is higher than 8000. The extra 200 kb/s aren't going to give you any noticeable improvement, so they're hardly worth the possible compatibility problems with old players.

If you want the absolute best video quality and you know your player(s) won't choke on video bitrates above 8000, then use compressed audio (ex., at 384 kb/s) and you can push the video up to 9400 kb/s.

Most commercial DVDs are encoded with a maximum video bitrate of 8000 or less, and an average between 4000 and 6000. But film compresses better than video (less noise, no interlacing), so for video you should use slightly higher values for the average (and also for the maximum, if you have a decent recent player).